The applicant proposed in Japanese Patent Publication (A) No. 2007-303423 a spark ignition type internal combustion engine which is provided with a variable compression ratio mechanism able to change a mechanical compression ratio and a variable closing timing mechanism able to change a closing timing of the intake valve, wherein at the time of engine low load operation, the mechanical compression ratio is raised compared with engine high load operation, to make an expansion ratio 20 or more.
In such a spark ignition type internal combustion engine, at the time of engine low load operation, the mechanical compression ratio (expansion ratio) is made 20 or more and the closing timing of the intake valve is made a timing away from intake bottom dead center so as to maintain the actual compression ratio relatively low compared with mechanical compression ratio, suppress the occurrence of knocking due to the actual compression ratio becoming higher, and while doing so realize an extremely high heat efficiency.
In this regard, if reducing the amount of intake air flowing into a combustion chamber by retarding the closing timing of the intake valve so as to move away from intake bottom dead center, part of the intake gas which is once sucked into the combustion chamber is pushed by the rising piston and blown back into an engine intake passage. The amount of blowback of the intake gas to the inside of the engine intake passage becomes greater the more the closing timing of the intake valve is retarded. Further, the strength of blowback of the intake gas to the inside of the engine intake passage becomes stronger the more the closing timing of the intake valve is retarded. In the spark ignition type internal combustion engine described in Japanese Patent Publication (A) No. 2007-303423, sometimes the closing timing of the intake valve is made extremely retarded. In such a case, the amount of blowback of the intake gas becomes extremely great, and the strength of blowback of the intake gas becomes extremely strong.
Under the conditions that the blowback of intake gas is large and strong in this way, if an exhaust gas recirculation (EGR) operation is carried out to make a part of the exhaust gas flow again into a combustion chamber, for example, the distribution of EGR gas among the cylinders will deteriorate, and the extent by which deposits stick will vary among cylinders.
That is, if the blowback of intake gas to the inside of the engine intake passage is large and strong, part of the intake gas will be blown back into the surge tank (that is, the collective part of the intake branch tubes). In this case, the part of the intake gas which was blown back into the surge tank will be sucked into not the original cylinder, but for example a cylinder which adjoins the original cylinder or a cylinder which is in the middle of a intake stroke when the intake gas is blown back into the inside of the surge tank.
If performing EGR at this time, EGR gas will be contained in the intake gas which is blown back into the engine intake passage. Therefore, if the blowback of intake gas to the inside of the engine intake passage is large and strong, part of the EGR gas will be sucked into not the original cylinder, but a cylinder which adjoins the original cylinder or a cylinder which is in the middle of a intake stroke when the intake gas is blown back into the inside of the surge tank. The amount of the EGR gas which is sucked into a cylinder different from the original cylinder changes depending on the flow of intake gas in the surge tank, the sequence by which the intake stroke is performed, etc. Therefore, there will be cylinders where the amount of EGR gas in the intake gas becomes larger and cylinders where the amount of EGR gas in the intake gas becomes smaller.
As a result, the amount of EGR gas will vary among the cylinders. Along with this, the air-fuel ratio will vary among the cylinders. Furthermore, the larger the amount of flow of EGR gas, the easier it is for deposits to stick to the wall surfaces of the intake ports. Therefore, in cylinders where the amount of EGR gas becomes large, the amount of deposits which stick to the wall surfaces of the intake ports will become greater, while in cylinders where the amount of EGR gas becomes small, the amount of deposits which stick to the wall surfaces of the intake ports will become smaller, and as a result, among the cylinders, the intake resistance to intake gas will differ. If variation occurs among the cylinders in the air-fuel ratio and intake resistance in this way, deterioration of the combustion and deterioration of the fuel efficiency will be invited.